It has been recognised that it is advantageous to monitor and measure the flow rate of air into a powered air respirator. This is because monitoring of the flow rate enables checks to be made to ensure that the filter flow rate capability is not exceeded, thus ensuring that the filter effectively filters all contaminants from the breathable air delivered to the wearer. Furthermore, the air delivery control system can using a closed loop flow measurement system maintain a given flow rate to the user irrespective of changes in conditions such as changes in the filter resistance or resistance of different headtop's or mask's worn by the user; changes in ambient air pressure resulting from, for example, variations in altitude and the like. Similarly, an active system can use the closed loop flow rate control to automatically compensate for the effects of breathing that constantly vary the load on the breathing air delivery fan assembly.
Modern respirators typically have a wide selection of accessories in the form of filter types, varying design of headtop or mask for different applications etc, and this gives rise to issues with maintaining a given flow rate across any combination of accessories. The atmosphere is, by its very nature, compressible, and thus as flow rates increase in a powered air respirator, the air compresses and a non linear relationship exists for flow rate and impellor speed. The flow resistance of a particular filter can also vary through its life as airborne particles are drawn into and retained on the filter medium, interfering with the movement of air therethrough and thus constantly increasing resistance through the filter life. Furthermore, the mere act of the user breathing causes variations in the load on the motor causing fluctuations in the flow rate. As the user breathes in, air is drawn through the filter, assisting the motor in supplying air and therefore effectively reducing the load. On the other hand, as the user breathes out, air flow is inhibited, even when a non-return valve is used, thereby increasing the load on the motor
It is relatively easy to monitor the speed of the impeller during operation of the apparatus, and it has been suggested to use this to calculate the flow rate. However, the natural efficiency of a fan design varies with different loads placed on it by varying resistances on the air inlet side and on the loads on the air outlet side. As a result, the impellor speed has no direct correlation to the flow rate.
Despite this issue with correlation between impellor speed and flow rate, for simple devices, such as a face mask with a built in fan and a limited choice of filter/s, it has been found that a very good approximation of flow rate can be made by comparing motor speed and motor load to create an algorithm of motor speed and load to estimate flow rate. It has also been known in the art to extend this approach to more complicated systems, but has meant that for a given filter or head top, the flow rate supplied by the respirator would be nearly constant but different for each combination. This approach also suffers from the drawback that it does not cater for changes resulting from variations in altitude (motor speed will increase as the density of the atmosphere decreases). In some models, these drawbacks can be alleviated by varying the number of filters that can be fitted in an attempt to reduce some of the filter resistance loading—as more filters are fitted, the filter resistance becomes lower and a balancing act commences on the number of filters to level out the inlet load on the respirator.
Another known approach is to increase all the lower load filters to the equivalent highest load filters thereby equalising the input load. The added complication to all this is that as the resistance changes over the life of the filter, the load increases complicating the load and speed algorithm. Another approach which has been proposed is to calibrate the device using a rotometer, which is primarily a mass flow meter and thus varies with humidity and altitude effects on density.
Thus it can be seen that prior art approaches have generally concentrated on approximation methods for flow rate and these have generally been found to be acceptable. However, as a result of filters getting more technical for different contamination challenges, a desire to maintain a flow rate regardless of the initial filter resistance and variations in filter resistance through its operating life, and a desire for different headtops to suit different applications, these approximation methods are now becoming insufficient.
Furthermore, analysis of the air within a typical powered air breathing apparatus assembly has shown that turbulence exists in the respirator assembly at flow rates that are required for breathing. This complicates any generally acceptable way of measuring airflow without the use of laminar air control system in place that causes a heavy load on the system either causing flow rates to be decreased and/or battery life of battery powered respirators to be reduced significantly.